The present invention relates to a print control device for a printer, typewriter or similar equipment and, more particularly, to a print control device capable of maintaining the absolute printing amount constant.
Generally, a printer, typewriter or similar equipment includes a sheet transport arrangement for feeding a sheet to a platen, and a sheet sensor for determining a condition in which the sheet is transported. The sheet sensor may be implemented by a mechanical switch having a lever to be operated by a sheet being transported. Alternatively, the sheet sensor may be comprised of a transmission type or a reflection type sensor having a light emitting element and a light-sensitive element. The transmission type sensor determines that a sheet is present when light issuing from the light emitting element toward the light-sensitive element is intercepted by the sheet, while the reflection type sensor determines so when light issuing from the light emitting element is reflected by the sheet onto the light-sensitive element. Further, the presence of a sheet at the time of sheet insertion and the absence of a sheet after printing may be determined by a single switch or by independent switches as desired. It is a common practice to feed, after the insertion of a sheet has been detected, the sheet to a position where the first line is to be printed, print data on the sheet, and then allow data to be printed even after the end of the sheet has been detected, i.e., from the position where the end of the sheet has been detected to the trailing edge of the sheet. For this purpose, the lines fed from the sheet detection position to the trailing edge of the sheet are counted and then compared with the absolute amount of line feed from the sheet detection position to the trailing edge of the sheet which is determined beforehand. Printing is allowed until the count reaches the absolute amount of line feed. This prevents data to be printed after a sheet has moved away from a print head and allows data to be printed up to the trailing edge of a sheet even after the end of sheet has been detected.
The problem with the conventional print control device is that the time when the end of a sheet is detected is apt to fluctuate depending on the color, thickness and hardness of a sheet to be used and due to a change in the speed of a sheet passing the sheet sensor. In the light of this, it has been customary to regulate the transport of a sheet for a moment by a feed roller after the sheet has moved away from the sheet sensor, and then feed the sheet again by the feed roller. As a result, although the sheet sensor does not need high accuracy regarding the feed of a sheet to the first line printing position, the change in the time when a sheet detection signal appears has influence on the absolute printing amount in the direction of lines of a sheet. Hence, if the accuracy of end-of-sheet detection is low, the absolute printing amount changes in the direction of lines.
Moreover, the sheet sensor of the type using a light emitting element and a light-sensitive element noticeably changes the output level thereof due to the aging of the elements and depending on the color of a sheet, distance to a sheet, etc. This causes the time when an end-of-sheet signal actually appears to deviate from the time when it should appear. Although the mechanical switch is inexpensive, the ON/OFF response thereof greatly changes due to the mechanical operation. In addition, the mechanical operation brings about chattering with the result that the time when the switch should generate a signal is delayed.
The deviation of the time when the switch detects a sheet from the time when the sheet is fed gives rise to the following problem. Since print data and line feed commands from a host CPU (Central Processing Unit) are not uniform, the time when an end-of-sheet signal appears apparently differs from, for example, the case wherein a sheet is once brought to a stop in the vicinity of the switch and then moved away from the switch to the case wherein it is continuously moved away from the switch at high speed. This problem becomes more serious as the sheet feed speed increases. Specifically, assume that a printing amount from the position where the end of a sheet is detected to the trailing edge of the sheet is determined with the above-mentioned position used as a reference. Then, the deviation of the time when an end-of-sheet signal appears changes the absolute number of lines which can be printed on a sheet and changes the distance between the last printing line to the trailing edge of the sheet.